1. Field of the Invention
The present invention relates generally to electrophotographic devices and electrophotographic printing methods, and more particularly, to a method and system for correcting a linearity error in the electrophotographic devices.
2. Description of the Related Art
An electrophotographic device produces an image or a copy of a document containing text, graphics, or a combination thereof. Examples of the electrophotographic devices include, but are not limited to, laser printers and laser copiers. The electrophotographic device produces the image by causing light to affect an electrostatic charge distribution on a photoconductive surface. The device includes a Photo Conducting (PC) drum (also referred to as photoconductor drum) which acts as an image-carrier and is coated with a photoconductive material. The device also includes a light source such as a laser beam or an array of Light Emitting Diodes (LED). The surface of the PC drum is exposed to the light from the light source to neutralize the electrostatic charge on the non-image parts of the PC drum. The density of the electrostatic charge on the surface of the PC drum is altered in the areas exposed to the laser beam relative to the areas unexposed to the laser beam. This process leaves a static electric-charged image on the surface of the PC drum to lift the toner particles. The toner particles are then transferred to different print media by electrostatic attraction.
The laser beam is directed towards the PC drum by passing the laser beam through a series of lenses and mirrors. The laser beam is first directed towards a rotating polygon mirror, which further directs the beam towards a series of lenses and mirrors. The laser beam, the rotating polygon mirror, and the series of lenses and mirrors are collectively known as a Laser Scanning Unit (LSU). The polygon mirror rotates with the help of a motor known as polygon mirror motor.
The laser beam moves across the surface of the PC drum in a uniform manner. For every small increment in the motion of the polygon mirror, the laser beam moves the same distance on the PC drum. The PC drum keeps rotating and the laser beam sweeps across the PC drum at an angle, to compensate for the motion of the drum, and makes a sweep straight across the page. A stream of rasterized data, including a series of 0's and 1's, is used to turn the laser on and off across the PC drum.
The laser beam that has a linear scan motion scans or sweeps the surface on a row-by-row basis. It moves linearly across the page and is turned on or off depending on the data bits (either one or zero). The laser beam takes more time to copy the image to the edges of the document, as compared to its center. Due to the difference in the speed of the laser beam (or beam velocity), a linearity error is generated. Further, manufacturing tolerances and imperfections of optical devices used by the LSU also generate the linearity error. The linearity or beam location or scan-location-error of the LSU describes the manner in which the laser beam moves across the PC drum when the rotating mirror spins at a fixed angular speed.
In a typical LSU, the rate at which the laser beam moves across the PC drum is relatively constant, since the polygon mirror rotates at a constant velocity. In such a system, when the laser is activated with a fixed time between activation periods, i.e., the time of turning the laser ‘on’ is fixed, the image containing equally spaced marks is printed correctly. This system is characterized by very low (<1 percent) linearity error. However, electrophotographic scanners and copiers with linearity error less than one are costly. Therefore, it is desirable to reduce the cost of such systems and also make sure that the linearity error is minimized.
To reduce the cost of the electrophotographic device, a set of lenses used to guide light beams on the PC drum can be removed from the electrophotographic device. However, for such low-cost electrophotographic devices, the linearity error increases to 15 percent or more. As a result of high linearity error in such systems, the placement of evenly spaced marks on the page is not accomplished by firing the laser at fixed time intervals. Various techniques are employed to ensure that the images are printed without any defects. Typical techniques used to correct the low linearity error include the use of a non-constant pel clock or a constant pel clock with ‘off’ slice insertion. The use of such techniques to correct the increased linearity error results in producing print artifacts in the form of narrow streaks in the print output obtained from the electrophotographic device.
In light of the facts given above, there is a need for a system and a method to correct the linearity error of the electrophotographic devices with a reduced number of lenses. Further, such a method and system should remove the print artifacts and also prove to be cost effective.